The present invention relates to encoding servo control information on a surface to control an apparatus which reads changes of state from the surface. More particularly, the present invention relates to angle encoding additional information within existing amplitude encoded position information.
Surfaces containing radially concentric tracks are used to record and store data for later retrieval. The data is stored along the tracks as a series of changes of state. One example of such a surface is a magnetic disc used in computer disc drives. Data is stored on a magnetic disc by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values.
To recover the data, a reading apparatus must sweep the surface along a specific track. Before the reading apparatus may properly recover the data it must first be correctly positioned over a desired track.
Positioning of the reading apparatus is performed through an iterative process. First, the reading apparatus is positioned somewhere on the disc. Then the reading apparatus reads control information from the disc at that position. From the control information, a controller determines the position of the reading apparatus on the disc. The controller then adjusts the position of the read apparatus based upon the desired location for the read apparatus and the determined position. At its new position, the reading apparatus again reads control information to determine if the move was successful. The process is repeated until the desired location is reached.
In the past, the control information was divided into two separate series of pulses. One series was used to obtain a track identifier (track ID) and a rotational position for the current position of the reading apparatus. The second series was used to determine a position within a track. The division of this information into two separate series is an inefficient use of servo control information space. Inefficient use of the servo control information space reduces the amount of data which may be stored on the surface. Thus, the current method of storing servo control information reduces the amount of data which may be stored on the surface.
In addition, the rotational position and track ID are presently stored using various codes including Gray codes. In order to decode the track ID and rotational position, the decoder must first know the frequency at which the coded information was written. This is accomplished by adding a series of equally spaced pulses before the track ID and rotational position information. This series of pulses, referred to as a synchronization field, is used by the decoder to determine a frequency for expected pulses. Based on that frequency, the decoder determines the existence or non-existence of pulses within the track ID and rotational position fields. The differences between the series of expected pulses and the series of received pulses represent the encoded information. The addition of the synchronization field before the track ID or rotational position increases the size of the servo control information area and thus reduces the amount of data which may be stored.